The instant invention relates to a process and apparatus for the electrolytic production of halogens and alkali metal hydroxides from aqueous alkali metal halide solutions. More particularly, it relates to the electrolysis of brine in a three compartment membrane cell in which the anode and cathode electrodes are physically bonded to the permeselective membranes.
It is now well known to electrolyze brine and other halides in electrolytic cells containing anode and cathode compartments separated by a liquid and gas impervious permselective membrane. The voltages required for electrolysis of halides in a membrane cell are, however, relatively high; one of the reasons being that the anode and cathode electrodes are physically separated from the permselective membrane. This introduces IR drops due to the layers of electrolyte between the membrane and the electrodes and IR drops due to gas blinding effects as bubbles of evolved chlorine and hydrogen gas are formed between the electrochemically active gas evolving electrodes and the membrane.
In an application, Ser. No. 922,316 filed July 6, 1978, in the names of LaConti, et al, assigned to the General Electric Company, the assignee of the present invention, a process for producing alkali metal hydroxides and halogens is described in which the electrochemically active anode and cathode electrodes, in the form of bonded porous masses of electrocatalytic and polymeric particles are bonded directly to and are embedded in the membrane to form a unitary electrode-electrolyte structure. Substantial reductions in cell voltages are realized because electrolysis occurs essentially at the interface of the bonded electrode and the membrane, and electrolyte IR drops and the IR drops due to gas blinding effects are minimized. Good contact must be maintained between the anode and cathode current collectors and the bonded electrodes in order to minimize ohmic losses at the collector/electrode interface. In the aforesaid application Ser. No. 922,316, and other cells of this type the current collectors are clamped between the housing and membrane to maintain good contact by mechanical, hydraulic or other clamping means.
In accordance with the present invention, Applicants have found that excellent contact at the electrode/current interface may be maintained and ohmic losses at the interface minimized by utilizing a three compartment cell in which the center or buffer compartment is operated at a positive pressure with respect to the other compartments. This forces the unitary membrane/electrode structure against the current collectors establishing uniform, constant, and controllable contact pressure thereby resulting in optimum cell voltages.
In addition to lowering the cell voltage required for halide electrolysis, the cathodic current efficiency at high caustic concentrations can also be increased substantially because a substantial portion of back migrating hydroxyl ions are discharged from the buffer compartment as sodium hydroxide. This reduces back migration of OH.sup.- ions through the anode membrane substantially. Improvement in current efficiency may therefore, be achieved by producing sodium hydroxide at a lower concentration in the buffer compartment along with highly concentrated caustic in the cathode compartment. Concentrated caustic can now be produced using cathode membrane with relatively low hydroxyl ions rejection characteristics and low electrical resistance without affecting the overall current efficiency. This is achieved, by in effect, incorporating multiple hydroxide rejection stages in a single cell process.
In preferred embodiments of the invention the permselective membranes, are hydrolyzed copolymers of polytetrafluoroethylene and perfluorosulfonylethoxy vinyl ethers having equivalent weights of in the range of 900-1700. Two such permselective membranes are utilized along with an outer housing frame to form the buffer compartment between anode and cathode compartments. The buffer compartment is operated with a pressurized distilled water or dilute caustic cathode feed thereby forcing the membranes outward into firm contact with the current collectors in the anode and cathode compartments.
Electrolysis of brine with cell voltages of 3.3 to 3.5 volts at 300 ASF with current efficiencies of 90% or more are readily achievable using permselective cathode membranes which have relatively low hydroxyl rejection characteristics and low electrical resistance.
It is, therefore, a principal objective of this invention to provide a three compartment electrolytic cell and a process for generating halogens and alkali metal hydroxides therein while minimizing cell electrolysis voltages.
Another objective of the invention is to provide a three compartment electrolytic cell and an electrolysis process carried out therein in which the buffer compartment is operated at a positive pressure differential to maintain uniform, constant and controllable contact between electrodes physically bonded to permselective cell membranes, and current collectors associated therewith.
Still another objective of the invention is to provide a highly efficient three compartment electrolytic cell and a process for generating chlorine and caustic in which the cell electrolysis voltage is minimized by maintaining uniform, constant and controllable contact pressure between electrodes bonded to the membranes and current collectors through a buffer compartment operated at a positive pressure with respect to the other compartment.
Other objectives and advantages of the invention will become apparent as the description thereof proceeds.
The objectives and advantages of this invention are realized by providing an electrolytic cell having a pair of permselective membranes, preferably cation membranes, which divide the cell into an anode, cathode, and buffer chambers. The two gas and liquid impervious permselective membranes have electrodes bonded to those surfaces which face the anode and cathode chambers respectively. The electrodes which are bonded masses of electrochemically active and polymeric particles, are bonded to and embedded in the surface of the membrane. Current collectors which are connected to an electrolysis voltage source are positioned in physical contact with the electrochemically active electrodes. Distilled water or a dilute solution of caustic is introduced into the buffer compartment as a positive pressure with respect to the anode and cathode compartments. The positive pressure forces the membranes outward into firm contact with the current collectors thereby maintaining a uniform constant contact pressure which minimizes ohmic losses between the current collector and the electrode. By maintaining a positive pressure differential of at least 0.5 psi and up to 5 psi; and preferably in a range of 1-2 psi, electrolysis cell voltages in the range of 3.35 to 3.5 volts at current densities of 300 ASF foot are readily achievable and represent voltage improvements ranging from 0.6 to 1.5 volts over conventional three compartment cells operated at 300 ASF.